Removed the warnings that are emitted when the LRU caches employed
by the mapper as well as loader srtategies reach their threshold; the
purpose of this warning was at first a guard against excess cache keys
being generated but became basically a check on the "creating many
engines" antipattern.   While this is still an antipattern, the presense
of test suites which both create an engine per test as well as raise
on all warnings will be an inconvenience; it should not be critical
that such test suites change their architecture just for this warning
(though engine-per-test suite is always better).

Change-Id: I41ef8cd642d05a845f53119b196440f9d7879cd9
Fixes: #4071

An :class:`.InvalidRequestError` is raised when a :func:`.synonym`
is used against an attribute that is not against a :class:`.MapperProperty`,
such as an association proxy.  Previously, a recursion overflow would
occur trying to locate non-existent attributes.

Change-Id: If2ce38c429a69951df4c94b71b74edbd59d775e3
Fixes: #4067

Fixed race condition in ORM identity map which would cause objects
to be inappropriately removed during a load operation, causing
duplicate object identities to occur, particularly under joined eager
loading which involves deduplication of objects.  The issue is specific
to garbage collection of weak references and is observed only under the
Pypy interpreter.

Change-Id: I9f6ae3fe5b078f26146af82b15d16f3a549a9032
Fixes: #4068

Fixed bug in :meth:`.Session.merge` where objects in a collection that had
the primary key attribute set to ``None`` for a key that is  typically
autoincrementing would be considered to be a database-persisted key for
part of the internal deduplication process, causing only one object to
actually be inserted in the database.

Change-Id: I0a6e00043be0b2979cda33740e1be3b430ecf8c7
Fixes: #4056
(cherry picked from commit 5243341ed886e10a0d3f7fef8ae3d071e0ffdcf0)

Change-Id: Ida0d01ae9bcc0573b86e24fddea620a38c962822

Implement rowcount assertions and single row check
for post_update as well as deletes.

Change-Id: I4e5ba7e8747bf0e0b41f569089eb8cdbf064b7a9
Fixes: #4062

Fixed bug where an :func:`.undefer_group` option would not be recognized
if it extended from a relationship that was loading using joined eager
loading.

In particular we need to double check the column both in terms of
the given "adapter" as well as without applying the "adapter"
when searching for the column in the result.

As we now avoid redoing the row processor step we also
improve on callcounts in joined eager loading.

Change-Id: I0f48766f12f7299f4626ff41a00bf1f5bfca5f3b
Fixes: #4048

Fixed regression where the use of a :func:`.undefer_group` option
in conjunction with a lazy loaded relationship option would cause
an attribute error, due to a bug in the SQL cache key generation
added in 1.2 as part of :ticket:`3954`.

Change-Id: Icd9a34f0b5aa96d6433a2ab9c8d3eaee0006f609
Fixes: #4049

Fixed regression introduced in 1.2.0b1 due to :ticket:`3934` where the
:class:`.Session` would fail to "deactivate" the transaction, if a
rollback failed (the target issue is when MySQL loses track of a SAVEPOINT).
This would cause a subsequent call to :meth:`.Session.rollback` to raise
an error a second time, rather than completing and bringing the
:class:`.Session` back to ACTIVE.

Fixes: #4050
Change-Id: Id245e8dd3487cb006b2d6631c8bd513b5ce81abe